In recent years, the environments of wells for petroleum or natural gas are becoming increasingly severe, and therefore the corrosion of oil well pipes for pumping crude oil from the ground or piping used to transport crude oil without being treated to suppress corrosion is becoming a major problem.
In the past, since Cr-containing steels have good corrosion resistance, a 13Cr martensitic stainless steel (0.2% C-13% Cr) has mainly been used in oil wells for crude oil containing large amounts of carbon dioxide gas. In wells for crude oil including not only carbon dioxide gas but further including minute amounts of hydrogen sulfide, due to the high sensitivity to sulfide stress corrosion cracking of the above-mentioned 13Cr martensitic stainless steel, Super 13Cr steel, which is a low-carbon, Ni- and Mo-added steel (0.01% C-12% Cr-5 to 7% Ni-0.5 to 2.5% Mo), was developed, and the scope of application of this steel is increasing.
However, in environments in which crude oil contains still larger amounts of hydrogen sulfide, sulfide stress corrosion cracking occurs even with Super 13Cr steel, and it has been necessary to employ a dual phase stainless steel, which is a premium grade of steel. Dual phase stainless steels have the problem that cold working is necessary in order to obtain a high strength, thereby making their manufacturing costs high.
It is predicted that increasing the added amount of Mo is effective for increasing the corrosion resistance of a martensitic stainless steel to hydrogen sulfide. In fact, based on experimental data for such steels which are actually used, it is indicated that the corrosion resistance in an environment containing a minute amount of hydrogen sulfide is improved by increasing the added amount of Mo.
FIG. 4 of CORROSION 92 (1992), Paper No. 55 by M. Ueda et al. shows that the rate of corrosion in an environment containing a minute amount of hydrogen sulfide is markedly reduced and the susceptibility to sulfide stress corrosion cracking is decreased by increasing the added amount of Mo. However, it also suggests that if the added amount of Mo exceeds 2%, the effect on improving corrosion resistance has a tendency to reach a limit and that a further significant improvement cannot be obtained.
Probably due to the influence of such experimental facts, the added amount of Mo is at most about 3% in martensitic stainless steels which have been put to actual use.
In patent documents as well, there are not a small number of disclosures of martensitic stainless steels to which a large amount of Mo is added. For example, JP 02-243740A, JP 03-120337A, JP 05-287455A, JP 07-41909A, JP 08-41599A, JP 10-130785A, JP 11-310855A, and JP 2002-363708A disclose martensitic stainless steels having a high Mo content. However, in these patent documents, there are no specific embodiments in which corrosion resistance, and particularly resistance to sulfide stress corrosion cracking, is improved if the Mo content is further increased compared to existing martensitic stainless steels to which at most about 3% Mo is added. Thus, there is no disclosure in these patent documents of technology in which marked improvements in resistance, such as resistance to sulfide stress corrosion cracking, can be achieved by increasing the Mo content. Accordingly, it cannot be said that there is a disclosure in the prior art of a steel having improved resistance to sulfide stress corrosion cracking compared to existing Super 13Cr steel.
JP 2000-192196A discloses a steel with a high Mo content to which Co is further added with the object of obtaining a martensitic stainless steel having the same level of corrosion resistance as a dual phase stainless steel. In the examples, it is described that this steel exhibits the same level of corrosion resistance as a dual phase stainless steel. However, its chemical composition includes not only a high level of Mo but also contains Co, which is an element which is normally not contained in a stainless steel. Therefore, it is difficult to say that the corrosion resistance is greatly improved just by the increase in the Mo content, and it is necessary to also take into consideration the effects of Co. Co is an expensive element, and the addition of Co may possibly make a martensitic stainless steel more expensive than a dual phase stainless steel, thereby offering problems with respect to its practical application.
JP 2003-3243A discloses a steel to which a large amount of Mo is added, but which has been tempered to precipitate an intermetallic compound composed primarily of a Laves phase in order to obtain a high strength. Namely, in order to obtain the same corrosion resistance as a Super 13Cr steel and to further increase strength, the amount of added Mo is increased for the purpose of achieving precipitation strengthening. However, even if the added amount of Mo is increased, if Mo precipitates as an intermetallic compound, an improvement in corrosion resistance cannot be expected.